1. Technical Field of the Invention
The invention disclosed and claimed herein generally pertains to a method and apparatus for estimating the signal-to-interference ratio (SIR) of signals transmitted to a receiver through an air interface. More particularly, the invention pertains to a method and apparatus of the above type wherein accuracy is significantly improved by employing received data symbols as well as pilot symbols in the SIR estimation. Even more particularly, the invention pertains to a method and apparatus of the above type wherein delay time encountered in the transmission power control loop is determined and used in SIR estimation.
2. Description of Related Art
As is well known by those of skill in the art, power control is important for capacity and efficiency in mobile code division multiple access (CDMA) systems. For example, if a mobile transmission/receiver unit is located close to a base station transmitter/receiver, the power level of signals transmitted to the mobile unit from the base station, in the absence of adjustment, would be comparatively high. This could interfere with transmissions to other mobile units located farther from the base station. Conversely, the signal power of transmissions from a mobile unit which was located far from the base station could be comparatively weak. Accordingly, it has become common practice to provide a base station with a transmission power control (TPC).
Currently, power control is accomplished by estimating the signal-to-interference ratio (SIR) of received signals. If the SIR of a signal received at a mobile unit is lower than a threshold value, a command or adjustment signal is sent to the transmitting base station to increase transmission power. The command is sent on the reverse link of the communication system, which could be either the uplink or the downlink, depending on which link is controlled. If the estimated SIR is higher than the threshold value, a command to decrease transmission power is sent. In a common arrangement for mobile transmission systems, signals are transmitted during each of a succession of time slots, each slot containing a block of pilot symbols and a block of data symbols. In such arrangement, the SIR estimation is carried out by using received pilot symbols, which are known in advance to the receiver. However, comparatively few of the transmitted signals comprise pilot symbols. Thus, a significant limitation is placed on the accuracy of the SIR estimation.
In one approach to overcome this limitation, the SIR is estimated from both pilot symbols of a transmitted slot, and data symbols of the preceding slot. However, in an arrangement of the above type transmission power is changed at the beginning of each slot. This power change must be known, in order to use pilot and data symbols from different slots in an SIR measurement. Moreover, since power changes at the transmitter are made in response to power control commands generated by the receiver, as described above, a finite period of time is required to send power control commands to the transmitter back from the receiver, and to then change power levels in response thereto. If this time period, referred to herein as delay time, is known, the change in power level between two particular slots can be readily determined at the transmitting base station. Such power change information is generally available, since it is retained after responding to successive power control commands. However, frequently the delay time will not be known. For example, it may not be known whether the delay is a single slot delay xcex941 or a two-slot delay xcex942. Also, the time delay can change if the mobile unit is moved from one cell to another. In either case, uncertainty as to the delay time will generally prevent using pilot symbols from one slot with data symbols from another slot to estimate SIR, since it will be very difficult or impossible to determine power change therebetween, so that compensation can be made in the estimation.
The invention is generally directed to an improved technique for estimating SIR of signals transmitted between a base station and a mobile unit of a mobile communication system, wherein the signals comprise pilot and data symbol blocks transmitted during each of a succession of time slots. The method of the invention includes the step of receiving the signals of a specified one of the slots, as well as the signals of the slot immediately preceding the specified slot, after transmission through an air interface. The method further comprises the steps of computing the delay time of associated power control commands sent back to the transmitter from the receiver; and estimating the SIR of signals of the specified slot from both the pilot block of the specified slot, and from the data block of the immediately preceding slot. A component or factor is incorporated into the SIR estimate which compensates for a change in power level between the specified slot and the immediately preceding slot, wherein the compensating component is determined from the time delay. The SIR estimate thereby becomes more accurate, and thus enables more accurate adjustment of transmission power.
In a preferred embodiment of the invention, the time delay is employed to identify or select the correct value for the power change between the specified slot and the immediately preceding slot from a succession of power changes, which have previously been generated and are respectively retained at the base station. Preferably, the method further comprises receiving the signals of each slot in an initial group of slots, wherein the transmission of the initial group precedes transmission of the specified slot, and then providing a SIR estimate and corresponding power change for each slot of the initial group. The SIR estimates for the slots of the initial group are determined only from respective pilot blocks thereof. The delay time is then computed by correlating the SIR estimates of the initial slot group with their respectively corresponding power changes.
In a very useful embodiment, respective signals of the initial slot group and the specified slot are transmitted from the base station and received at a mobile unit. However, in an alternative embodiment the signals are transmitted from a mobile unit, and received at the base station.
In a further useful embodiment, the invention is used in connection with a communication system employing CDMA. However, the invention could also be used in a time division multiple access (TDMA) system.